Over the past twenty years, the use of gas turbine systems has been on the increase among utilities, as well as in industry. Gas turbine systems, whether used in conjunction with steam turbines or alone, provide their users with an efficient source of energy, mechanical or electrical, at a modest capital cost. Typically, capital cost is one-half to one-third that of more conventional steam-powered plants. Other advantages associated with gas turbine systems stem from their availability at modest sizes when compared to steam turbines, the relatively fast delivery time to the user, and the flexibility that a modular approach can provide to satisfying increasing energy demands.
The working fluid for gas turbine systems is directly derived from the products of combustion of carbonaceous fuels. The highly stressed working elements (blades) of the turbine need to preserve their shape and integrity for efficient operation over protracted periods of time. In consequence, exacting requirements are usually imposed upon the fuels which can be used to generate the working fluid for such systems. Up until now, non-solid fuels, such as natural gas or oil, have been required to provide a working fluid of a purity which minimizes deposition, erosion, and corrosion of the turbine elements.
In recent years, oil prices have increased by about a factor of ten. Many electric utility and industrial plants are caught in a cost squeeze. Restriction to the use of gas or oil as the fuel for the operation of gas turbine systems, makes such systems less attractive to the user. Coal and related solid carbonaceous fuels are our most abundant reserve of fossil fuels. Accordingly, efforts are presently underway to develop systems which would make possible the use of coal in gas turbine systems. Such systems, if economical, would be very attractive to users in the utility, industrial, and transportation markets.
Two approaches aimed at using coal with gas turbine systems have received attention. The first uses a process in which coal is first gasified, and the products of gasification, after being suitably cleaned of impurities at moderate or low temperatures, are made available for use in the gas turbine system. Drawbacks of this approach are reduced efficiency and a substantial increase in capital cost associated with the gasification and clean-up processes.
The second approach relies on coal beneficiation. This approach involves removal of the major fraction of the coal impurities. Typically, a reduction of the mineral matter content to levels below one-half of one percent by weight of the coal, and comminution to sizes small enough so that, after combustion, residual ash particles will not exceed dimensions of the order of five microns, are required. Removal of a major fraction of the sulfur in the coal is also desirable to eliminate the need for back-end SOx removal equipment. This approach promises to preserve the low capital cost of existing gas turbine systems, but the high cost of chemical beneficiation of coal would result in fuel costs comparable to oil.
The present invention is directed to the resolution of the problem by a process in which clean-up of the working fluid is carried out in conjunction with its generation. With this approach, the benefits of modest capital and fuel costs are retained. The present invention utilizes a slagging combustor in combination with other equipment and operating regimes to provide a working fluid of desired purity for direct use in gas turbine systems.
Slagging combustors are described in U.S. Pat. No. 4,217,132 to Burge, et. al. and U.S. application Ser. No. 788,929 filed Oct. 18, 1985, now U.S. Pat. No. 4,685,404 which is a continuation of application Ser. No. 670,417, each assigned to the assignee of record and each incorporated herein by reference.
We have found that the aforementioned slagging combustion systems can provide the following advantages. High power density: about 1.0 million Btu/hr per cubic foot of volume and per atmosphere of pressure in the primary combustion chamber of the slagging combustor. High carbon conversion: conversion of substantially all carbon to oxides of carbon within the combustion system. Removal of non-combustibles: Capture and removal from the gaseous products of combustion of most, of the order of 95 percent, of the non-combustible mineral content of the fuel before the fluid leaves the slagging combustion system. Low NOx: Low nitrogen oxide emissions achieved by fuel and air staging for fuels naturally containing substantial amounts of nitrogen. Low SOx: Control of sulfur oxide emission by the addition of suitable gettering agents into the slagging combustion system. Thermal efficiency: Delivery to the end-use equipment of a gaseous working fluid having about 85 to 95 percent of the chemical potential energy of the carbonaceous fuel. Durability: Protection of the walls of the high temperature primary combustion chamber by a layer of slag so that deleterious corrosion and/or erosion of the walls can be kept within commercially acceptable limits.
The present invention is directed to improved apparatus utilizing a slagging combustor and a process which generates a high-purity working fluid from carbonaceous fuels, such as coal, for use in gas turbine systems.